PALOMAR 300A METER BULB

[Sonar]

I'm not familiar with the light bulb that illuminates the meter but I'm assuming it's the older type of bulb that were used in flashlights. Definitely not an LED type. It's been blinking off then back on and ocastionaly dims without going out (weather I'm transmitting or not). Then there's times it won't dim or blink for the whole time I'm using the amp (45 minutes.) But for the most part it blinks 80% of the time it's on. Does anyone know if this bulb is secured in a socket or soldered in? The bulb itself has been dim as long as I could remember. Should I replace it or just wait until it stops working all together? Before anyone gets perturbed and says "why don't you just remove the cover and see for yourself". Being disabled is a bitch sometimes. Having to reply on other's is a part of my life I'm used to but having to ask my wife or someone else to do something that's not nessesry is something I avoid. Thanks. 73

 

[peabody830]

I would say its on its way out, question is , is it going to drive you nutts while it does. lol

 

[Robb]

Sounds like a bad/intermittent connection.
If it were bad; then it wouldn't work at all . . .

 

[Sonar]

I would say its on its way out, question is , is it going to drive you nutts while it does. lol

Negative. It's not bothering me at all but it would be nice to see what a new bright bulb looks like in there. I've seen severall YouTube clips of these 300a's and they all look to give off a dim yellowish glow. I own two and although I haven't used the other one in years I recall that bulb looking the same. I should've read the other replies first. I'm trying to find out if the bulb is in a socket or the + - leeds are solder directly to the bulb. If I don't get an answer here I'm going to have the yl remove the cover so I can get a look see. Whether it's shattered and or in a socket once the covers off I will remove the bulb. I'm sure I can find a replacement bulb on eBay. Thank you my friend.73

 

[tecnicoloco]

has a twist on socket

 

[Sonar]

has a twist on socket

Thanks. Now that I know I will have the yl romove the chrome cover, then the bulb. I'm hoping there's something printed on the bulb itself making it easy to look online for a replacement. If not I would assume it is or once was commonly used in many items of the era and will be easy to locate a replacement. Thanks again. 73

 

[Sonar]

Update on the 300a lightl bulb issue The build began blinking more frequently. Suddenly the tubes wouldn't lite at the same time the builb went out. I thought this is much more than an original build on its last leg. I checked the fuse in the RF deck. It was good. . I moved on to the fuse in the 300a's power supply. Although it wasn't blown, it was rusty and one of the fuses metal caps was loose. It was the rusty one that was loose. The fuse aperared to be original to the power supply. I also noticed that the fuse receptical was just as rusty as the fuses cap. Luckily I had a 15 amp in my vihicles glove compartment. I sprayed a small amount of droxit before putting the new fuss in. Whalah!! The tubes lit upon powering up the amp and after about 45 minutes with my eye on the bulb, it didn't blink once. I left the amp on for an hour before keying it up and all is fine. I did retune it as it has been quite a while and with 2 watts dead key and 15 pep from the sonar It's dead keying 90 and pep is a hair under 500 (after over coupling). I said it once and will again, "these Palomar 300a's have to be one of the best CB 11 meter amps from that time period". Reliable as hell with 500 watts is all I ever asked for in a CB base amplifier. I would absolutely get another. Besides the high temps reached because of the amps cover being in close proximity to the tubes I can't think of another complaint. I've seen pics of some owners using 4 metel brackets to raise the cove allowing for greater air flow lowering the high temp reached inside the amp. It doesn't look prity but for the sake of the components I might do the same. Thanks. 73

 

[nomadradio]

The fuse in the power transformer cabinet protects the 12-Volt AC secondary. Older ones didn't have it. Palomar found that a short to the 12 Volts would not draw enough fault current to blow the main fuse in the RF cabinet, the one that protects the AC line cord. Added a 15-Amp fuse to the transformer cabinet to prevent warranty returns.

We started adding that fuseholder to any 300A we worked on, just to keep from being blamed if a 12-Volt short were to croak the transformer.

That fuse is prone to heat problems. It runs hot when everything is okay. If any oxide or schmoo build up on the end caps of the fuse, it can trip out from the heat alone, even if there is no fault current to trip it the normal way. There is solder up inside the metal caps. If the solder connection to the fuse element goes bad, the fuse may look perfectly okay but cut in and out under load.

Another case of "terminal crud".

73

 

[Sonar]

The fuse in the power transformer cabinet protects the 12-Volt AC secondary. Older ones didn't have it. Palomar found that a short to the 12 Volts would not draw enough fault current to blow the main fuse in the RF cabinet, the one that protects the AC line cord. Added a 15-Amp fuse to the transformer cabinet to prevent warranty returns.

We started adding that fuseholder to any 300A we worked on, just to keep from being blamed if a 12-Volt short were to croak the transformer.

That fuse is prone to heat problems. It runs hot when everything is okay. If any oxide or schmoo build up on the end caps of the fuse, it can trip out from the heat alone, even if there is no fault current to trip it the normal way. There is solder up inside the metal caps. If the solder connection to the fuse element goes bad, the fuse may look perfectly okay but cut in and out under load.

Another case of "terminal crud".

73

Thanks nomadd. That's exactly what happened. IMO it took 30+ years for that to happen. I'm convinced that fuse was original. I think you did a terrific job helping to build/design this amp. I'm very impressed with the 300a. I don't think I'll ever enjoy using any other 11 meter amp from this time. It's nice to look at and works. And works. And works. I did change out everything that one should with any amp of this age and no amp of that time period is going to be problem free forever and if something should happen (knock on wood) I would have no problem spending the money (within reason) to get up and running properly again. Thanks for the interesting info. 73

 

[nomadradio]

Uh, wait a minute.

"helping build/design this amp"?

I'm old, but not that old. I was a young service tech when that one first hit the market, but I never worked for Palomar. Haven't even been to the San Diego area where it was made.

But I've definitely acquired a familiarity with in the 40-plus years since they were first sold. I do remember it didn't take long for people to start blowing them up.

73

 

[Sonar]

Uh, wait a minute.

"helping build/design this amp"?

I'm old, but not that old. I was a young service tech when that one first hit the market, but I never worked for Palomar. Haven't even been to the San Diego area where it was made.

But I've definitely acquired a familiarity with in the 40-plus years since they were first sold. I do remember it didn't take long for people to start blowing them up.

73

Lol. Sorry about that nomadd.

 

[Sonar]

Uh, wait a minute.

"helping build/design this amp"?

I'm old, but not that old. I was a young service tech when that one first hit the market, but I never worked for Palomar. Haven't even been to the San Diego area where it was made.

But I've definitely acquired a familiarity with in the 40-plus years since they were first sold. I do remember it didn't take long for people to start blowing them up.

73

Hello nomad. I've been looking for the old post about the 300 a and how to tell the difference between which one will work off which power supply and which one will not. My power supply and RF deck work perfectly together and were originally purchased from the first owner. There is no doubt in my mind that my 300a and its power supply were sent from the factory at the same time to the original owner and are perfectly matched for each other. Now I have come across another white 300a RF deck (white) but I cannot locate the post in which you explain how to tell which 300a power will work with which RF deck? Can you please explain to me (again) how to tell if the rf deck I acquired without a power supply will work with my current 300a's power supply? What is it I should exactly be looking for under the cover of the new 300 a that will tell me it's either an OK match for my power supply or not? Thank you very much nomadd. 73.

 

[nomadradio]

A 300A built for the higher-voltage 560-Volt transformer had a single square-block rectifier bridge with four legs on it, next to the two HV filter caps. This is a 1500-Volt rated part and hard to find a direct replacement.

A RF deck built for the lower 280-Volt transformer has four conventional-looking 3-Amp rectifier diodes on the board with the two big HV filter caps.

Telling the transformers apart is a different matter. Safest way is to come up with 12 Volts AC. Feed that into the 120-Volt input on the transformer. A meter on the HV side of the transformer will show 28 Volts if it's the one meant for the doubler circuit that has four single 2-leg diodes. And if it shows 56 Volts it is meant for the bridge-rectifier HV circuit. This keeps all the exposed voltages low while making a measurement. Safer for the meter, too.

73

 

[Sonar]

A 300A built for the higher-voltage 560-Volt transformer had a single square-block rectifier bridge with four legs on it, next to the two HV filter caps. This is a 1500-Volt rated part and hard to find a direct replacement.

A RF deck built for the lower 280-Volt transformer has four conventional-looking 3-Amp rectifier diodes on the board with the two big HV filter caps.

Telling the transformers apart is a different matter. Safest way is to come up with 12 Volts AC. Feed that into the 120-Volt input on the transformer. A meter on the HV side of the transformer will show 28 Volts if it's the one meant for the doubler circuit that has four single 2-leg diodes. And if it shows 56 Volts it is meant for the bridge-rectifier HV circuit. This keeps all the exposed voltages low while making a measurement. Safer for the meter, too.

73

Thanks nomadd. It's definitely not going to work with my power supply. To bad he lost the power supply. The deck is in very original condition. His and mine have different rectifier sections. 73

 

[nomadradio]

The RF deck can be modified to match the higher-voltage transformer. Remove the diodes from the voltage-doubler circuit. Building a bridge circuit will require using eight 1N5408 rectifiers, with two of them in series in each of the circuit's four legs. Kinda cramped, but doable.

The single-piece black-epoxy bridge rectifier that Palomar used was rated at 1500 Volts. Haven't seen that part advertised in decades. This does also necessitate putting a separate 270k (or so) bleeder resistor across each of the two HV filter caps. They serve both to bleed off stored charge when power is removed AND to divide the voltage equally 50/50 between the two caps.

Never have seen a "how to" for this on line.

A bit risky if you have to write your own instructions to do it.

73